This invention relates to chromatic dispersion in optical systems, and more particularly to detection or discrimination of chromatic dispersion in optical signals used by optical transmission systems.
Current optical fibers have a property known as chromatic dispersion which causes light transmitted along the fiber to experience an amount of propagation delay that is dependent on the frequency, or wavelength, of the light. Optical signals transmitted over these fibers by optical transmissions systems are modulated carrier signals which have a bandwidth determined by the upper and lower modulation sidebands. Different frequency components of an optical signal will experience different amounts of propagation delay, depending on the frequency (or wavelength) of the component, as the optical signal travels along the fiber. The resulting variance in propagation delays among the different frequency components changes the optical signal, thereby making error-free demodulation of the signal more difficult.
At a particular frequency, an optical fiber has a xe2x80x9cnullxe2x80x9d point at which the propagation delay is minimum. Since dispersion is defined as the change in propagation delay relative to frequency (or wavelength), the dispersion at the null point will be zero and it will be opposite in polarity on either side of this point. The positive dispersion of one type of fiber can be used to approximately compensate for the negative dispersion of another type of fiber, and in this way optical links can be engineered to have minimal dispersion over a narrow frequency (or wavelength) range.
However, in dense wavelength division multiplexed (DWDM) systems, which typically have anywhere from 40 to 160 DWDM optical signals modulated on carriers spaced apart at 50-100 GHz and using optical carriers in the 1520-1580 nm range, engineering optical links to provide minimal dispersion for all of the DWDM optical signals is difficult, if at all possible, because of the wide range in frequency (or wavelength) of the signals. Typically, the amount of dispersion imparted on a group DWDM optical signals transmitted over an optical link will vary largely across the range of signals. When these DWDM signals are switched with optical signals from other optical links having different dispersion characteristics, the result is a new group of DWDM optical signals having an even wider, and now non-systematic variance in dispersion across the range of signals. This result is most prevalent in versions of automatically switched optical networks (ASON) which use purely photonic switches (as opposed to electro-optical switches and transponders) because the links over which optical signals travel between source and destination nodes in the network changes dynamically to adapt to changing traffic demands placed on the network.
Therefore, it would be desirable to have a means of detecting the amount of dispersion in optical signals received over a dispersive optical link or at least discriminating which polarity of chromatic dispersion is present, thereby allowing the correct amount of dispersion compensation to be applied to each optical signal, in either an open-loop (magnitude/polarity detection) or closed loop (residual polarity detection) application.
It is an object of the present invention to provide a chromatic dispersion discriminator for determining the amount of chromatic dispersion in optical signals.
The invention uses the chromatic dispersive properties of two or more different types of optical fibers in order to determine the polarity and magnitude of dispersion that a received optical signal has undergone as a result of being transmitted over one or more dispersive optical links.
Embodiments of the invention offer the advantage of allowing dispersion detection or discrimination to be performed on a per wavelength basis which is the first step to enabling compensation to be performed on individual optical signals on the basis of the amount of dispersion each optical carrier signal has undergone during transmission over an optical link, thereby allowing for more accurate dispersion compensation as compared to means employing engineered links which provide predetermined dispersion compensation.
According to an aspect of the present invention there is provided a dispersion discriminator for determining the amount of dispersion in an amplitude modulated optical signal. The amplitude modulated optical signal is a double side band signal produced from on-off, or quasi on-off, amplitude modulation used in conventional optical systems. The dispersion discriminator includes: a splitter for dividing the optical signal into at least first and second portions; a first dispersion leg for causing a first additional amount of dispersion in the first portion; a second dispersion leg for causing a second additional amount of dispersion in the second portion that is opposite in polarity and substantially equal in magnitude to the first additional amount such that the amount of dispersion in the second portion is detectably different from the amount of dispersion in the first portion; and a dispersion detector for receiving the first and second portions from the respective dispersion legs, determining the amount of dispersion in the optical signal by detecting a difference between the amount of dispersion in the received first and second portions, and providing an indication of said amount of dispersion in the optical signal.
Conveniently, the dispersion detector includes: a first spectrum detector for determining the power spectrum of the modulation sidebands of the received first portion; a second spectrum detector for determining the power spectrum of the modulation sidebands of the received second portion; and a dispersion processor for comparing the power spectra of the received first and second portions in order to determine the difference between the amount of dispersion in the first and second portions. Each spectrum detector includes an optical receiver combined with a scanning filter for determining the power spectrum of the respective portion of the optical signal provided to it.
Where a dispersion processor is provided, the dispersion processor may be operable to compare xe2x88x923 dB cut-off frequencies of the power spectra (which results from the upper and lower sideband components at this frequency having been each shifted 45 degrees in opposite directions) in order to determine the difference between the amount of dispersion in the received first and second portions. Additionally, the dispersion processor may be further operable to compare notches and peaks in the power spectra in order to determine the difference between the amount of dispersion in the received first and second portions. In particular, theory indicates that a first strong or deep cancellation or notch with increasing modulation frequency will occur at a frequency equal to two times the xe2x88x923 dB cut-off frequency (which results from the upper and lower sideband components at this frequency having been each shifted 90 degrees in opposite directions). At this point one leg of the discriminator should show a full, enhanced or attenuated but non-zero signal spectral density, whilst the other leg would show zero output, with a dispersed signal input, but both legs would show a xe2x80x9cnullxe2x80x9d at the same point if the input signal had zero dispersion.
Where the dispersion detector includes spectrum detectors, each spectrum detector outputs a signal representing a normalized power spectrum of an optical input signal provided thereto. The optical input signal for the first spectrum detector is the first portion of the optical signal input into the dispersion discriminator after the first portion has been modified by the incremental dispersion of the first arm of the discriminator, and the optical input signal for the second spectrum detector is the second portion after it has been modified by the incremental dispersion of the second arm of the discriminator.
Where each spectrum detector outputs a signal representing a normalized power spectrum, the dispersion processor includes a comparator for providing a comparison of the signals representing the normalized power spectrum, and a spectrum-dispersion correlator for determining, from the comparison, the polarity and approximate magnitude of the dispersion in the optical signal. Each spectrum detector may include: an optical receiver for receiving the optical input signal by demodulating data carried in the optical input signal and outputting the demodulated data in an electrical signal; a scanning band-pass filter for selectively passing, in a filter output signal, a band of frequency components of the electrical signal provided thereto; a power measuring device for providing a measurement of the total power of the electrical signal; another power measuring device for providing a measurement of the power of the filter output signal; and a divider for dividing the measurements from the power measuring devices and outputting the signal representing the normalized power spectrum.
According to another aspect of the present invention there is provided a dispersion discriminator for determining the amount of dispersion in an amplitude modulated optical signal from two or more portions of the optical signal. The dispersion discriminator includes: a first dispersion leg for causing a first additional amount of dispersion in a first portion of the optical signal; a second dispersion leg for causing a second additional amount of dispersion in a second portion of the optical signal, the first and second additional amounts being opposite in polarity; and a dispersion detector for receiving the first and second portions from the respective dispersion legs, determining the amount of dispersion in the optical signal by detecting a difference between the amount of dispersion in the received first and second portions, and providing an indication of said amount of dispersion in the optical signal.
The dispersion detector may be further operable to receive a third portion of the optical signal and compare the received first portion relative to the received third portion with the received second portion relative to the received third portion in detecting said difference. In such a case, the dispersion detector includes: a first spectrum detector for determining the power spectrum of the received first portion; a second spectrum detector for determining the power spectrum of the received second portion; a third spectrum detector for determining the power spectrum of the received third portion; and a dispersion processor. The dispersion processor is operable to: determine a first relative power spectrum that is the power spectrum of the received first portion relative to the power spectrum of the received third portion; determine a second relative power spectrum that is the power spectrum of the received second portion relative to the power spectrum of the received third portion; and compare the first and second relative power spectra in determining the difference between the amount of dispersion in the received first and second portions.
According to still another aspect of the present invention there is provided a dispersion discriminator for determining the polarity and magnitude of dispersion in an amplitude modulated optical signal. The dispersion discriminator comprises: an input port for receiving the optical signal; an output port for transmitting an output signal indicating the polarity and magnitude of dispersion in the optical signal; a splitter having an input coupled to the input port and having at least two outputs, the splitter operable to divide the optical signal into at least first and second portions; a positive dispersion leg having two ends, one of which is coupled to one output of the splitter, the positive dispersion leg being operable to add an amount of positive dispersion to the first portion; a negative dispersion leg having two ends, one of which is coupled to the other output of the splitter, the negative dispersion leg being operable to add an amount of negative dispersion to the second portion; and a dispersion detector having an output coupled to the output port and having two inputs, each input coupled to a respective other end of the positive and negative dispersion legs, the dispersion detector being operable to determine the polarity and magnitude of dispersion in the optical signal by comparing the first and second portions received from the first and second dispersion legs and to transmit an indication of the polarity and magnitude of dispersion in the optical signal via the output signal.
Other aspects of the invention include combinations and sub combinations of the features described above other than the combinations described above.